In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition. In LTE networks, Physical Downlink Control Channel (PDCCH) is used for downlink scheduling. In the current LTE specification, PDCCH can be configured to occupy the first one, two, or three OFDM symbols in a subframe.
One promising technology for LTE is the use of Multiple Input Multiple Output (MIMO) antennas that can further improve the spectral efficiency gain by using spatial division multiplexing. Multiple antennas allow for an additional degree of freedom to the channel scheduler. Multi-user MIMO (MU-MIMO) is considered in LTE Rel-10. As compared to Single-user MIMO (SU-MIMO), MU-MIMO offers greater spatial-domain flexibility by allowing different users to be scheduled on different spatial streams over the same RB. By scheduling the same time-frequency resource to multiple UEs, more UEs will be scheduled in the same subframe to take advantage of spatial multiplexing. To enable MU-MIMO, individual control signaling must be indicated to each UE via PDCCH. As a result, more PDCCH transmissions are expected, as the number of scheduled UEs per subframe will increase. However, the maximum 3-symbol PDCCH region may not be enough to accommodate the increased number of UEs in LTE. Due to limited control channel capacity, the MIMO performance degrades because of non-optimized MU-MIMO scheduling.
In LTE Rel-11, various deployment scenarios for coordinated multi-point (CoMP) transmission/reception are introduced. Among the different CoMP scenarios, CoMP scenario 4 refers to Single Cell ID CoMP in heterogeneous network with low-power remote radio heads (RRH). In CoMP scenario 4, low-power RRHs are deployed within the macrocell coverage provided by macro-eNB. The RRHs have the same cell IDs as the macrocell. In such single cell ID CoMP operation, PDCCH must be transmitted from all transmission points and then soft combined without cell-splitting gain. Because the physical signal generation of PDCCH is linked to cell ID, UEs served by different points can only share the same physical resource for PDCCH if the same cell ID is shared among the different points. This creates a control channel capacity problem similar to the MU-MIMO situation illustrated above.
To address the control channel capacity problem, an UE-specific downlink scheduler for MU-MIMO/CoMP has been proposed. In LTE, it extends the PDCCH design to a new X-PDCCH, which is in the legacy Physical Downlink Shared Channel (PDSCH). How to signal UEs about the scheduling information of X-PDCCH, however, is unclear. For example, if the signaling is provided by PDCCH for each UE, then the same control channel capacity problem occurs. On the other hand, if the signaling is configured by higher-layer, then control overhead of X-PDCCH cannot be adjusted dynamically. A solution is sought.